Now click on Movies - you will see the exported Trailer with a name like large.m4v, medium.m4v, 720p.mov or similar.
I meant to add that it's best if you copy the movie from within the Package Contents folder, then paste it to the Desktop. It's important not to move things from Package Contents folders. You can then import the copy of the exported Trailer from the Desktop to the iMovie Event, as I described. Following that, it's OK to trash the Desktop file, as it will now reside in the iMovie Events folder.
Note also that, if you use "Export using QuickTime" you can nominate where the shared file is to be located, for example, the Desktop or any folder of your choosing.
John